The present invention relates generally to turbines and particularly to land-based gas turbines for power generation employing closed-circuit steam-cooling paths for cooling the hot gas components and particularly relates to a bore tube assembly facilitating the supply of cooling steam to the hot gas components and return of the spent cooling steam.
Steam cooling of hot gas path components, for example, the buckets of a gas turbine, has been proposed in the past and found efficacious in land-based power generating plants. While gas turbines are typically air-cooled, for example, jet engines employ compressor discharge air for cooling the hot gas components, steam cooling is more efficient in that the losses associated with the use of steam as a coolant are not as great as the losses realized by extracting compressor bleed air for cooling purposes. Also, in combined cycle operations, steam cooling is particularly advantageous because the heat energy imparted to the steam as it cools the gas turbine components is recovered as useful work in driving the steam turbine in the combined cycle operation.
In U.S. Pat. No. 5,593,274 of common assignee herewith, there is disclosed a gas turbine having coaxial steam passages for supplying cooling steam to hot gas components of the rotor, for example, the buckets, and returning that spent cooling steam to a return. Various refinements and improvements, however, in the supply and return of the steam for cooling purposes are provided by the present invention.
The present invention provides a bore tube assembly for supplying cooling steam to the hot gas components of the rotor and returning the spent cooling steam in a manner to promote efficiency of the closed-circuit steam path. Various aspects of the bore tube assembly contribute to that improved efficiency. For example, in accordance with the present invention, the bore tube assembly includes a pair of concentric tubes defining a coolant steam supply passage between the outer and inner concentric tubes and a spent cooling steam return passage within the inner tube. The bore tubes form part of the rotating structure of the rotor. Steam is supplied to the supply passage by a steam gland having labyrinth seals on opposite sides of the inlet to the outer tube. The forward ends of the inner and outer tube are coupled to an end cap for turning the axially supplied annular flow of cooling steam in a radial outward direction for delivery to the steam-cooled buckets and turning the spent cooling steam flowing radially inwardly from those buckets in an axial direction for flow through the inner tube to a return. The end cap includes first and second axially spaced sets of a plurality each of circumferentially spaced openings in respective communication with the steam supply passage and spent cooling steam return passage. The first and second sets of openings of the end cap communicate with first and second axially spaced sets of a plurality each of circumferentially spaced, radially extending tubes carried by the rotor for respectively distributing the cooling steam to the steam-cooled buckets and conveying the spent cooling steam from the buckets through the end cap and bore tube assembly to the return. The end cap affords a unique steam flow transition between the radial outer components of the rotor and the bore tube assembly.
Apart from the end cap itself, another aspect of the present invention includes an inner core within the end cap. The inner core has a shaped head or body for directing the spent cooling steam returning from the steam-cooled buckets radially inwardly through the tubes into the axially directed return passage of the inner tube of the bore assembly. The inner core also carries a plurality of vanes for removing any tendency of the returning cooling steam to swirl in the axial return flow passage within the inner tube. That is, the vanes remove the swirling components of flow of the steam and direct the steam substantially in an axial direction.
In another aspect of the present invention, a radiation shield overlies at least a portion of the outer tube between it and the aft shaft to minimize heat transfer from the steam supply passage to the aft main bearing. The shield per se resists thermal radiation to the aft main bearing which might otherwise obtain a temperature above acceptable limits for the bearing pad and oil film of the bearing. Additionally, an air gap is provided between the bore tube and the radiation shield, enabling the shield to provide thermal resistance to heat transfer by conduction. The radiation shield is secured at one end to the outer bore tube, while the other end remains free for axial thermal expansion.
Another aspect of the present invention resides in the provision of a strut ring between the inner and outer tubes of the bore tube assembly which enables thermal expansion and contraction of the inner tube relative to the outer tube. The strut ring includes inner and outer rings, the outer ring preferably being secured by welding to the inner surface of the outer tube of the bore tube assembly. The inner tube is slidable relative to the inner ring of the strut ring to enable thermal axial expansion of the inner tube relative to the strut ring. The strut ring maintains the orientation, i.e., the concentricity of the inner tube relative to the outer tube. Additionally, the strut ring includes a plurality of struts extending between the inner and outer rings and which struts are canted off radii of the strut ring. The canting of the struts enables limited thermal radial expansion of the inner tube relative to the outer tube while maintaining concentricity of the inner and outer tubes. Further, the trailing edges of the struts are angled in an axial downstream direction to shed vortices. Other aspects of the present invention will become apparent upon a review of this specification.
In a preferred embodiment according to the present invention, there is provided in a turbine having a rotor rotatable about an axis including a plurality of turbine wheels mounting turbine buckets, a bore tube assembly for conveying a cooling medium to the buckets of at least one of the turbine wheels and conveying spent cooling medium to a return, comprising elongated outer and inner tubes spaced from one another and concentric about the axis defining first and second passages for respectively conveying the cooling medium in one axial direction and conveying spent cooling medium in an axial direction opposite the one direction, an end cap adjacent one end of the tube assembly having first and second sets of a plurality each of circumferentially spaced openings in communication with the first and second passages, respectively and first and second sets of a plurality each of circumferentially spaced radially extending passageways carried by the rotor in communication with the respective first and second sets of openings in the end cap for distributing the cooling medium to the buckets of the one turbine wheel and conveying the spent cooling medium through the end cap and bore tube assembly to the return.
In a further preferred embodiment according to the present invention, there is provided in a turbine having a rotor rotatable about an axis including a plurality of turbine wheels mounting turbine buckets, a bore tube assembly for conveying a cooling medium to the buckets of at least one of the turbine wheels and conveying spent cooling medium to a return, comprising elongated outer and inner tubes spaced from one another and concentric about the axis defining first and second passages for respectively conveying the cooling medium in one axial direction and conveying spent cooling medium in an axial direction opposite the one direction, first and second sets of a plurality each of circumferentially spaced generally radially extending passageways carried by the rotor in communication with the respective first and second passages for distributing the cooling medium to the buckets of the one turbine wheel and conveying the spent cooling medium through the end cap and bore tube assembly to the return and a bearing journal surrounding at least in part the outer tube, a radiation shield carried by the outer tube for thermally insulating the bearing journal against heat transfer by radiation from the cooling medium flowing through the passages.
In a still further preferred embodiment according to the present invention, there is provided in a turbine having a rotor rotatable about an axis including a plurality of turbine wheels mounting turbine buckets, a bore tube assembly for conveying a cooling medium to the buckets of at least one of the turbine wheels and conveying spent cooling medium to a return, comprising elongated outer and inner tubes spaced from one another and concentric about the axis defining first and second passages for respectively conveying the cooling medium in one axial direction and conveying spent cooling medium in an axial direction opposite the one direction, a strut ring disposed between the inner and outer tubes and having an outer ring and an inner ring interconnected with one another by a plurality of circumferentially spaced struts, one of the inner ring and the outer ring being fixed to one of the inner tube and the outer tube, respectively, with another of the inner ring and the outer ring and another of the inner tube and the outer tube being slidable relative to one another.
Accordingly, it is a primary object of the present invention to provide a novel and improved cooling circuit in the bore tube assembly of a turbine rotor enabling efficient supply of a cooling medium to selected hot gas components of the rotor and return of spent cooling steam.